1. Field of the Invention
The present invention relates to a nonaqueous electrolyte and a nonaqueous electrolyte secondary battery.
2. Description of the Related Art
Presently, a lithium ion secondary battery has been commercialized as a nonaqueous electrolyte secondary battery used in a portable apparatus such as a portable telephone. The lithium ion secondary battery comprises a positive electrode containing lithium cobalt oxide (LiCoO2), a negative electrode containing a graphitized material or a carbonaceous material, a nonaqueous electrolyte prepared by dissolving a lithium salt in an organic solvent, and a separator formed substantially of a porous film. A nonaqueous solvent having a low viscosity and a low boiling point is used as the solvent for preparing the nonaqueous electrolyte.
It is of high importance to decrease the thickness of the secondary battery in accordance with decrease in the thickness of the portable apparatus. In order to decrease the thickness of the secondary battery, it is necessary to decrease the wall thickness of the case housing the positive electrode, the negative electrode, the separator and the nonaqueous electrolyte. However, the lithium ion secondary battery using a case having a small wall-thickness gives rise to the problem that the case is swollen and deformed if a gas is generated from the negative electrode in the stage of the initial charging or if the positive electrode reacts with the nonaqueous electrolyte during storage of the secondary battery under high temperatures not lower than 60° C. so as to decompose the nonaqueous electrolyte by oxidation and, thus, to generate a gas. If the case is deformed, the secondary battery fails to be housed in the electronic apparatus, or malfunction of the electronic apparatus is brought about.
Jpn. Pat. Appln. KOKAI No. 4-14769 discloses an organic solvent containing as a main component a mixed solvent consisting of propylene carbonate, ethylene carbonate and γ-butyrolactone. It is taught that it is possible to improve the low temperature discharge characteristics of a cylindrical nonaqueous electrolyte secondary battery by using a nonaqueous electrolyte prepared by using the mixed solvent noted above, which contains 10 to 50% by volume of γ-butyrolactone.
On the other hand, Jpn. Pat. Appln. KOKAI No. 11-97062 discloses a nonaqueous electrolyte prepared by dissolving lithium tetrafluoro borate (LiBF4) in a mixed solvent containing 10% by volume of γ-butyrolactone. It is taught that the particular nonaqueous electrolyte permits suppressing the decomposition by oxidation of the positive electrode containing lithium cobalt composite oxide as an active material.
Further, a lithium ion polymer secondary battery comprising a polymer gel electrolyte is disclosed on page 23 of “Summary of Lectures for 67-th Meeting of Electrochemical Institute” published on Mar. 28, 2000. It is reported that the polymer gel electrolyte is prepared by polymerizing a mixed solution of an electrolyte and a polyfunctional acrylate monomer, followed by chemically crosslinking the resultant polymer. The electrolyte is prepared by dissolving a solute salt of LiBF4 or LiPF6 in a mixed solvent prepared by mixing ethylene carbonate and γ-butyrolactone in a mixing ratio by volume of 2:3.
However, the secondary battery disclosed in any of the three prior publications quoted above gives rise to the problem that, if the secondary battery is used under a high temperature environment, the negative electrode reacts with the nonaqueous electrolyte so as to bring about decomposition of the nonaqueous electrolyte by reduction, resulting in failure to obtain a long cycle life.
Still further, Jpn. Pat. Appln. KOKAI No. 2000-235868 is directed to a nonaqueous electrolyte secondary battery using a nonaqueous electrolyte comprising a nonaqueous solvent and a lithium salt dissolved in the nonaqueous solvent. It is taught that the nonaqueous electrolyte comprises over 50% by volume and 95% by volume or less of γ-butyrolactone based on the total amount of the nonaqueous solvent, ethylene carbonate, and at least one kind of a third solvent selected from the group consisting of propylene carbonate, vinylene carbonate, trifluoropropylene, diethyl carbonate, ethyl methyl carbonate and an aromatic compound.
However, the secondary battery comprising the nonaqueous electrolyte containing the nonaqueous solvent of the composition pointed out above gives rise to the problem that it is impossible to obtain a long cycle life under a high temperature environment.